The vertebrate central nervous system (CNS) is patterned by environmental signals that control the specification of cell fate. Understanding the mechanism of cell fate specification in the nervous system is vital for our ability to diagnose disease and design regenerative therapeutic treatments. One important signal in the developing CNS is produced by Wnts, which regulate transcription through the downstream effectors (i-catenin and Tcf. Wnt/B-catenin signaling plays an important role in spinal cord patterning, but the cellular and molecular targets of Wnt signaling in this tissue are unknown, and it is unclear whether Wnts act only to promote cell division or also directly assign cell fate. The zebrafish now gives us an ideal model system to address this problem. In this study, we will test the hypothesis that Tcf- mediated transcription directly regulates progenitor cell fate specification in the spinal cord. First, we will test whether Tcf7 and Tcf3 are required for the expression of spinal progenitor domain-specific genes. Our preliminary data suggest that at least one intermediate domain is mis-specified when Tcf3 activity is lost. We will now examine whether other progenitor domain markers also require Tcf3, whether dorsal progenitors specifically require Tcf7 activity, and whether these molecules function independently of cell-cycle control. Second, we will test whether Tcf3 normally acts as a transcriptional activator, represser, or both. We will examine whether Tcf3 overlaps with and is required for endogenous B-catenin activity, determine whether mutant forms of Tcf can phenocopy or rescue Tcf3 loss-of-function phenotypes, and ask whether Tcf3 functions synergistically or antagonistically to canonical Wnt signaling. These experiments will support either a model in which Tcf3 functions exclusively as a represser, or one in which it also activates target genes. Third, candidate target genes will be tested as direct transcriptional targets of Tcf3 signaling by chromatin immunoprecipitation (ChIP) analysis. We will use a combination of known genes, computational genomic analysis, and an unbiased screen to identify candidates. These experiments will yield a picture of Tcf3 targets in vivo, and the roles of the B-catenin signaling pathway in spinal cord progenitor specification. Overall, these studies will allow us to understand how genes are regulated during spinal cord development, ultimately resulting in the correct placement and wiring of spinal neurons. This understanding will help in the treatment and repair of spinal cord injuries and disease.